1. Field of the Invention
The invention relates to a compensation device and a power transmission system using such a compensation device. In particular, the invention relates to such compensation devices which comprise a transformerless reactive series compensator.
2. Description of the Related Art
Compensation devices are typically used in power transmission systems in order to increase the power transmission capacity and to make it more stable. Normally a power transmission line has a circuit breaker. When the circuit breaker is opened or closed, overcurrents and overvoltages are generated. In particular, in power transmission systems having a high power and a long power transmission line, such overvoltages and overcurrents are generated due to the parasitic inductances and capacitances of the power transmission line. Moreover, they can damage the compensation devices and the AC power systems connected to the power transmission lines.
The invention in particular relates to the problem of how the compensation devices can be efficiently protected from these overcurrents and overvoltages.
Several different examples of compensation devices have been used in recent years to provide more stable and higher efficiency power transmission systems. Such power transmission systems are called flexible AC transmission systems (FACTS). Examples of semiconductor devices applied to the compensation devices are self arc-suppressing semiconductors like gate-turn-off thyristors (so-called GTOs) and gate-commutated-turn-off thyristors (so-called GCTs). They have been applied to power converters, namely inverters, and it is envisaged that their application will be more widespread in the future in order to realize more stable power transmission systems.
There is one compensation device, which comprises a transformerless reactive series compensator (so-called TL-RSC), used in the FACTS devices.
FIG. 9 shows a power transmission system comprising two AC power systems 1a, 1b coupled to each other through power transmission lines 2a, 2b and a circuit breaker 4 provided between the AC power system 1b and the power transmission line 2b. A compensation device 3 (comprising a compensator unit CU) is provided between the power transmission lines 2a, 2b to increase the power transmission capacity, and to make it more stable. The compensation device 3, CU can comprise a transformerless reactive series compensator TL-RSC. As shown in FIG. 9 the TL-RSC 3 is connected in series to the transmission lines 2a, 2b which connect the AC power systems 1a, 1b.
FIG. 10 shows a transformerless reactive series compensator TL-RSC 3 as known from the prior art reference "Transformerless Reactive Series Compensators with Voltage Source Inverters" of Proceedings of the Power Conversion Conference (PCC)--Nagaoka 1997, PP. 197-202. In FIG. 10, self arc-suppressing semiconductors are designated by 5a to 5d, free-wheeling diodes connected in anti-parallel with each of the self arc-suppressing semiconductors 5a to 5d are designated by 6a to 6d, a single-phase inverter which consists of the self arc-suppressing semiconductors 5a to 5d and the free-wheeling diodes 6a to 6d is designated by 7, a DC capacitor of the inverter is designated by 8, filter reactors are designated by 9a, 9b, a filter capacitor is designated by 10 and a filter circuit is designated by 11. The self arc-suppressing semiconductor 5a is separated from the free-wheeling diode 6a. In recent years, however, reverse-conducting self arc-suppressing semiconductors which integrate both functions of the self arc-suppressing semiconductor 5a and the free-wheeling diode 6a in the same package have been developed. When the reverse-conducting self arc-suppressing semiconductors are applied, the free-wheeling diodes 6a to 6d are not necessary in FIG. 10.
As shown in FIG. 10, the single-phase inverter 7 is connected in series and indirectly with the power transmission lines 2a, 2b without any transformer. That is, the inverter 7 is coupled to the power transmission lines 2a, 2b via the filter circuit 11 which is serially inserted in the lines 2a, 2b. The filter circuit 11 essentially suppresses harmonic distortions of the power transmission systems which result from the single-phase inverter 7 being operated by a pulse width modulation (so-called PWM) control. A separate configuration of the filter reactors 9a,9b is not essential and it is possible to use only one of them.
The TL-RSC 3 has been proposed for new FACTs systems, but it has not been realized yet. So as to realize it, start-up and shut-down operations and/or another special operations of the TL-RSC 3 are very important problems to solve in particular, with respect to the size and weight of the individual components and with respect to the overcurrents and overvoltages occur when opening/closing the circuit breaker 4 as will be explained hereinafter.
Firstly, one case should be considered that the two AC power systems 1a, 1b are connected to the power transmission lines 2a, 2b. Here, it is assumed that the power transmission lines 2a, 2b mean long distance and high voltage power transmission lines on the order of 4000 kV and 500 km. As well-known, the power transmission lines 2a, 2b are described by at least an equivalent circuit as shown in FIG. 11. The equivalent circuit is expressed by a distributed constant circuit 15 which consists of internal resistances 12a, 12b, internal inductances 13a, 13b and a parasitic capacitance 14. Generally, the parasitic capacitance 14 exists mainly between the power transmission lines 2a, 2b and the earth GND. The power transmission lines 2a, 2b are freely divided by several distributed constant circuits 15.
When the power transmission lines 2a, 2b are connected by the circuit breaker 4, an overcurrent in much excess of the nominal current flows through the power transmission lines 2a, 2b. This disturbance of the power transmission lines 2a, 2b is a result of a resonance phenomenon generated by the internal inductances 13a, 13b and the parasitic capacitance 14. The disturbance is amplified if there is a large phase difference between both AC power systems 1a, 1b.
The overcurrent problem may be partially eliminated by using only the components shown in FIG. 10. In case the TL-RSC 3 is connected in series with the power transmission lines 2a, 2b, the overcurrent may pass through the single-phase inverter 7 or it may charge the filter capacitor 10. If the former case is allowed, the self arc-suppressing semiconductors 5a to 5d must turn off i.e., absorb the overcurrent, because the single-phase inverter 7 must protect the DC capacitor 8 from being charged to an overvoltage as a result of the overcurrent. Therefore, high current capacity self arc-suppressing semiconductors 5a to 5d are necessary. On the other hand, if the latter is allowed without turn-off operations of the self arc-suppressing semiconductors 5a to 5d, the overcurrent charges the filter capacitor 10 alternately. The single-phase inverter 7 can operate as single-phase diode rectifier so that the overcharging voltage charges the DC capacitor 8. If the overvoltages must be suppressed, both capacitances of the filter capacitor 10 and the DC capacitor 8 must be increased, which results in components of large size. Thus in this prior art TL-RSC, large size components are necessary to solve the problem of overcurrents/overvoltages.
In addition, the above-mentioned prior art reference describes the conventional application and proposes that larger compensatable capacity can be achieved when using a multiple TL-RSC 16 which consists of several cascaded TL-RSC units CU1 to CU3 connected in series. Each of the TL-RSC units CU1 to CU3 has the same configuration as shown in FIG. 10. Here, another problem ocurrs when one of the TL-RSC units CU1 to CU3 has a failure in the single-phase inverter 7. The multiple TL-RSC 16 must not stop operating easily because it must not only compensate the power transmission systems but also basically transmit the power from one side of the AC power system 1a (1b) to the other side. Therefore, it is essential that the multiple TL-RSC 16 shown in FIG. 12 have a continuous operation function. This demands that even a single-phase inverter 7 having a failure must not stop operating completely, because all the TL-RSC units CU1 to CU3 are connected in series with the power transmission lines 2a, 2b and the single-phase inverter 7 having a failure must continue to pass the line current. Therefore, a solution is necessary in which all the self arc-suppressing semiconductors 5a to 5d maintain an on-state by shorting the output-terminals of the single-phase inverter 7 having a failure.